This invention relates to sputtering apparatus for depositing material onto a workpiece.
It is well known that material can be sputtered from a target, ionised in a vacuum chamber and subsequently attracted to a workpiece, such as a semi-conductor wafer by means of electric fields.
A convenient means of ionising the sputtered material is to use an immersed coil, driven at radio frequencies, and such an arrangement is described in xe2x80x9cFundamental Characteristics of built-in high-frequency coil-type sputtering apparatusxe2x80x9d, J. Vac. Sci. Technol. A 7(2), March/April 1989. However to ionise a sufficiently high proportion of the sputtered material, a relatively high (xcx9c30 mT) chamber pressure is needed. This pressure, combined with the presence of the ionising RF coil, creates an unacceptably high non-uniformity in the deposition across the workpiece e.g. typically 20%-30% standard deviation of surface thickness. Although highly non-uniform, the deposition on the substrate can be engineered to have radial symmetry; the centre being thickest and the edge being thinnest. Such symmetry allows for the possibility of compensating for such non-uniformity by the addition of a secondary source of sputtered material positioned concentrically around the substrate. The second source will preferably deposit material at the substrate edge giving an overall thickness profile that is acceptably uniform e.g. a standard deviation of surface thickness  less than 8%.
A two part target is described in U.S. Pat. No. 4,606,806 and comprises an inner planar part and a frusto-conical outer part. In EP-A-0807954 the compensatory target comprises a solenoid annulus of target material which serves both as the ionising coil and the secondary sputter source. This arrangement has the considerable disadvantage that the coil/secondary target is very expensive to manufacture as the grade of metal has to be around 99.999% refined and it needs to be replaced frequently as it is consumed. The forming of such materials is very expensive and it is extremely difficult to cool them in an economic fashion. This is disadvantageous, because the resultant thermal stress results in flaking. Thus in EP-A-0807954 a shield ring is provided to prevent sputtering from the primary target onto the secondary target/coil. For many processes this works reasonably well, but problems arise in the common process requirement to deposit varied layers upon semi-conductor wafers consisting of Ti followed by TiN, preferably within the same chamber. The TiN is deposited by flowing small quantities of nitrogen during Ti sputtering processes. This causes Titanium target surfaces to nitride and Titanium Nitride is sputtered onto the substrate surface.
Ti and TiN have very different thermal expansion coefficients and any thermal cycling will create additional stresses causing flaking to be more likely, particularly from an un-cooled target coil. The arrangement of EP-A-0807944 does not overcome this problem sufficiently unless prohibitively long etching processes occur between the Ti and TiN processes.
The present invention sets out to mitigate at least some of these problems and certain embodiments provide significant improvements on all fronts.
From one aspect the invention consists in sputtering apparatus for depositing material onto a workpiece including a chamber, a target exposed in the chamber and a workpiece support located in the chamber opposite the target characterised in that the apparatus further comprises a re-sputtering surface disposed between the target and the workpiece of electrically conductive non-target material for receiving a coating of target material for re-sputtering onto the periphery of the workpiece to enhance uniformity of deposition and means for negatively biasing the surface to enable re-sputtering of the coating.
Preferably the re-sputtering surface is disposed generally circumjacent the support and the surface, or part of it, may be inclined towards the support. The re-sputtering surface may be in the form of a ring and the ring may be a frusto-triangle in cross section. The surface may be made of stainless steel or other suitable robust or easily formed material usable within a vacuum environment. The apparatus may further comprise an adhesion enhancing coating on the re-sputtering surface for enhancing the adhesion of target material to the surface for example a coating of molybdenum.
An RF coil may be disposed in the chamber between the target and the re-sputtering surface. In this case the RF coil is preferably formed of non-target material and has an external surface or surfaces for receiving a coating of target material to prevent sputtering of the non-target material. Conveniently the re-sputtering surface is located between the coil and the support so that it can sputter material onto that part of the coil which is in shadow with respect to the target.
The coil and/or the re-sputtering surface may be hollow to define a passage for liquid coolant therein and the coil may be made of stainless steel or other suitable material.
In any of these cases at least part of the coating on the re-sputtering surface and/or the coil may be provided by sputtering from the target during a pre-treatment operation of the apparatus, i.e. an operation which takes place prior to the positioning of a workpiece on the support.
This may preferentially take place simultaneously with the pre-sputtering of the target as is common practice prior to deposition upon a workpiece. A shutter is generally employed to avoid sputter material depositing upon the workpiece support.
The apparatus may include control means for operating the apparatus in the first pre-treatment mode, in which there is no workpiece, to coat the re-sputtering surface and/or the coil with target material for the subsequent process and in a second deposition mode in which target material is also deposited on the workpiece. The control means may include a computer program to calculate the necessary conditions for the first mode for any particular selected second mode operation.
From a further aspect the invention may consist in sputter apparatus for treating a workpiece including a chamber, a target disposed in the chamber, a workpiece support located in the chamber opposite the target and an RF coil disposed within the chamber between the target and the support characterised in that the coil is made from non-target material and carries, or is provided with prior to and/or during treatment of a workpiece, a coating of target material and in that a further target is located between the coil and the support for sputtering materials onto that part of the coil which is in shadow with respect to the first mentioned target.
The further target may be annular and may be generally a frusto-triangle in section. The coil and/or the further target may be hollow to define a passage for liquid coolant therein and each or either of them may be made of stainless steel. The coating on the coil may be provided at least partially by sputtering from the targets during a pre-treatment operation of the apparatus or, when the further target it itself made of non-target material and coated, from the first target primarily.
The coil is preferably operated at frequencies, powers and in modes that minimise its DC potential thus minimising its net sputtering thus ensuring it remains coated with target material throughout workpiece processing. It may be operated at net ground potential, segmented or in any way at as low a negative DC bias as is consistent with the process.
From a further aspect the invention consists in sputtering apparatus for depositing material onto a workpiece including a chamber and a target disposed in the chamber characterised in that the apparatus further comprises a re-sputtering surface of electrically conductive non-target material for receiving a coating of target material for re-sputtering and means for controlling the biasing of the surface such that a coating is formed on the surface and is also re-sputtered in a manner that there is always a coating of target material present, on substantially all the re-sputtering surface during the sputter deposition onto a workpiece.
The invention also consists in a method of operating sputtering apparatus including pre-coating elements of the apparatus which are made of non-target material so that the non-target material is not sputtered during treatment operation and/or the uniformity of deposition is enhanced. The invention may also include other steps set out above.
Although the invention has been defined above it is to be understood it includes any inventive combination of the features set out above or in the following description.